1. Field of the Invention
The present invention relates to Numerically Controlled (NC) machines, and more particularly to the drilling, reaming and countersinking of holes by such machines.
2. Discussion of Prior Art
NC machines are commonly used in many manufacturing applications, especially for forming edges, drilling holes and creating surface features. An article to be manufactured is designed for example using a computer software package, and then data defining the design is communicated to an NC machine, which is also given datum co-ordinates to allow the article to be manufactured accurately.
In aerospace applications, it is common to use NC machines to locate and drill holes for details and assemblies. Often for aerodynamic efficiency reasons, it is necessary to provide surface countersinking of holes for external panels and skins, so that the surfaces of fasteners when located in the holes are flush with the surface of the surrounding panels or skin.
To achieve the aerodynamic effect of a smooth surface, the depth of the countersink must be extremely accurate. Indeed, the tolerances generally required are such that the surface of the countersunk fastener must be within 0.1 mm of the surface of the surrounding panel or skin. It is very difficult to achieve these tolerances, as the surface of the panel or skin being drilled may be curved, may not be of uniform thickness or may have surface inconsistencies. The thickness of the material to be drilled may vary within the tolerance band throughout its length, but as tolerances have a cumulative effect, any deviation from the nominal thickness will result in the tolerance band of the countersink being reduced, making it even more difficult to achieve the desired result from the countersinking process.
To accurately drill and countersink, it is desirable to know the exact position of points on the surface of the material at which it is to be drilled. The NC machine is programmed with the x and y co-ordinates of the hole as well as the depth to which is to be drilled in the z direction, on the assumption that the surface of the material is regular and is to be found at a particular value of z with respect to the datum. However, if the material has a within-tolerance surface dip at the x, y co-ordinates given for drilling and countersinking, then the countersink tool will not penetrate far enough into the drilled hole to allow the surface of a fastener located in it to fit flush with the rest of the surface of the material. The fastener will instead stand proud of the surface of the material and may, in use, produce unwanted aerodynamic effects. Similarly, if the material has a slight surface rise at the given x, y co-ordinates, then the drilling and countersinking tools will penetrate too far into the material resulting in a hole that is too deep, so that the surface of a fastener located in it will be positioned below the surface of the material.
There are currently two known methods for establishing the variation of surface contour with the x, y position of points on the surface of a material to overcome the problems described above. The first method is to use a commercially available surface sensor unit to ensure that the hole is drilled and countersunk to the correct depth. Currently available surface sensors fit easily on to the arbor holding the cutting tool, and operate mechanically, generally either by compressing a spring or by moving a piston to effectively decouple the cutting tool from the arbor and thus allowing the arbor to move relative to the cutting tool. To achieve this, the sensor has a nose slidably located on the non-rotating housing around the cutting tool. As the drill moves forwards, upon touching the article being drilled, the nose ceases forward movement and the housing slides forwards relative to the nose and the cutter, decoupling the arbor from the cutter and thus preventing further drilling. The NC machine is programmed to retract at a position beyond the actual depth to be drilled but within the collapse range of the surface sensor.
These mechanical sensors tend to be heavy and bulky, and to be prone to mechanical problems and inaccuracies during drilling, because they need several moving parts to operate. The speed of rotation of the cutting tool is also limited by having the mechanical sensor on the arbor, as is the accuracy of drilling at high speeds.
It has been found that directly drilling and countersinking in one operation generally produces holes outside of the tolerances required. Accordingly, it is necessary to pre-drill the hole first and then ream countersink to the desired size as a second operation. The reamer follows the pre-drilled hole and consistently produces holes within tolerance. It is both costly and time consuming to employ two cutting tools, and the preferred way of manufacturing countersunk holes would certainly involve using a drill/countersink to produce the hole directly using one tool only.
The second method is to probe the surface of the material at every point in the x, y plane where a hole is to be located, to find out the exact local surface position in the z direction at each of these points. The variation between the measured surface position and the programmed values of the z co-ordinate at each point can then be added to the data used to control the NC machine. This allows a drill/countersink tool to be used to produce a finished hole needing only one tool, as preferred. Probing may be undertaken on a commercially available Co-ordinate Measuring Machine (CMM), used generally for inspection, and fitted with a sensitive probe capable of communicating its exact location. However, probing the surface of the material at every potential hole position is extremely time consuming, and not suitable for the production of objects having many holes, such as aircraft skins and panels. Additionally, if a CMM is used, this method requires that the article be removed from the NC machine and the holding tool used for its initial production and then be accurately placed on the CMM, to allow probing of the potential hole locations, before being fitted back into its holder and being accurately positioned again relative to the NC machine. This is a time consuming operation, and may generate further tolerance problems due to these repositioning operations.
Additionally, after manufacture, there is generally a requirement for the panel or skin to be inspected. This would usually involve the article being removed from the NC machine and placed on a CMM, for inspection by probe. The position of the holes and from this the general shape of the article can be sensed by the probe. The inspection, though usually necessary, is extremely time consuming.